Pyrotechnic compositions



Aug. 3, 1965 s. ZEMAN ETAL PYKOTECHN I C COMPOS ITIONS Filed Oct. :5. 1960 Fig.2

INVENTORS Samuel lemon Millard F. Scogqin,Jr. BY firlif/ rriga 5%41 ATTORNEYS 3,198,678 PYROTECHNIC COMPOSITIONS Samuel Zeman, Huntsville, andMillard F. Scoggin, Jr.,

Decatur, Ala., assignors to Thiokol Chemical Corporation, Bristol, Pa.

Filed Oct. 3, 1960, Ser. No. 60,134 1 Claim. (Cl. 149-44)- This invention relates to pyrotechnic compositions, and relates particularly to compositions suitable for use in electric squibs or. detonators.

Electric squibs or detonators are used in many fields requiring the generation of a small controlled explosion or of a flame for ignition or detonation of a larger explosive or combustible body. These squibs or detonators often rely upon the'fusion of a very fine wire, heated electrically, to ignite a thermally sensitive material sur rounding the wire. employed to ignite one or more intermediate charges to bring about detonation or ignition of a principal body of explosive or combustible material.

The necessity for rapid functioning of squib or detonator devices when even small electrical currents 'are passed through the igniter wire has necessitated the frequent prior art use in such devices of highly sensitive explosives in'combination with igniter wires of high resistance and small cross-section. of such devices by high temperatures, physical impact, or currents generated in the ignition wire by stray electromotive fields is large.

The recent demand for squibs or detonators for igniting the propellant charge in rocket motors, or for activation of explosive charges initiating stage separation, thrust reversal, destruction. etc., in rocket motors, has emphasized the need to have a highly safesquib or detonation device. The. accidental detonation of a squib in the ignition system of a large rocket motor, for example, could be catastrophic.

Thus, the design of squibs or detonators has recently been modified to make use of the so-called exploding bridgewire. current athigh current density through an igniter wire is used to cause rapid. explosive, deterioration of the wire. The explosion generated by such passage of a short duration pulse of high voltage and current through.

the ignition wire is used to ignite a pyrotechnic charge surrounding the wire, which charge can be further employed to trigger other charges in an ignition train;

The present invention concerns pyrotechnic. compo-- sitions for use in such an explodingbridgewire igniter. The compositions are such as will not ignite when the bridgewire is heated or fused by DC. voltages, static charges, radio frequency energies, or other extraneous sources of electrical energy which cause heating at a low rate. ofthe bridgewire or the pyrotechnic composition adjacent thereto. The compositions further are highly thermally stable, outstandingly immune to accidental ignition by impact, and are resistant to detonationeven by the discharge of low voltage pulses through the bridgewire.

The use of the pyrotechnic compositions of the invention in exploding bridgewire igniters permits the. functioning: of these igniters at times varying from 20 microseconds -to several milliseconds, depending on the: input energy, over a temperature range greater than from --70 F. to 160 F., and at altitudes of up to more than 300,000 feet.

A. better appreciation of the invention and of its many advantages can be had by reference to the accompanying The thus-ignited substance may be The danger of accidental ignition In these squibs, the sudden passage'of a technic compositions of the presentinvention are useful; and.

FIGURE' 2 is a diagram of a conventional electrical.

circuit suitable for use in exploding a bridgewire squib such as is shown in FIGURE 1.

In particular, FIGURE 1 shows an exploding bridgewire'squib comprisingbase plug 11, such'as of plastic, partially enclosed within cap 12, such as of metal, and enclosing lead wires 13 which are bridged within the squib. by bridgewire 14. Primary charge 15, comprising a compositionaccording to the present invention, is adjacent to bridgewire. 14, and is separated by thin, rupturable closure. 16, such as of aluminum-foil 1 mil thick, from secondary charge 17 within the squib. The secondary charge may comprise conventional ignition or detonating materials such as mixtures of boron and am monium perchlorate, aluminum and ammoniumperchlo-- rate, or lead styphnate. Insulating members 18, which. may be of nylon, protect lead wires 13 on their. entry and exit from base plug 11. Spacers 19, suitably of a material such as a phenolic resin, circumferentiallysurround primary charge 15 and secondary charge 17.

FIGURE 2 shows a circuit in which the bridgewire squib is represented schematically by bridgewire 21, which may be, for example, a fine gold wire 2Imils in diameter. The circuit also contains high voltage power source 22, for example generating. 1800-2300 D.C. volts, charging storage condenser 24, which may, for example, be of 1 microfarad capacitance, through resistor 23. Switch 25 permits charging of condenser 24, and ionization gap switch 26 permitsdischarge of condenser 2&- through bridgewire 21. In such a circuit, discharge of condenser 24 is achieved by supplying a trigger signal to gap switch 26, causing ionization and permitting cur rent to flow across the gap. Because of the low im= pedence and inductance of the circuit, a large surge of current can be made to flow through bridgewire 21, causing. it to vaporize with explosive violence. Insuch a circuit, the current flowing through the bridgewire may be at a current density greater than one million amperes per'square centimeter, and peak current is reached in less than five microseconds. The bridgewire itself may be of a. noble metal such as a platinum-iridium or platinumrhodium alloy, generally of a diameter of about 2 mils and between 0.05' and 1.10 inch in length.

The pyrotechnic compositions of the invention, which are used toadvantage'in the exploding bridgewire. squib, comprise a finely'divided metallic fuel andoneor more finely divided inorganic oxidizing agents, optionally in the;

presence of a finely divided combustible'synthetic resin which may function both as an oxidizing. agent and as a carrier for the finely divided inorganic: substances. 'As

a metallic fuel, a metallic aluminous metal such as aluminum or alloys of aluminum with magnesium, silicon,

.titanium, or other. readily oxidizable active metals canbe in the form of metal flakes, in. atomized form, or as a.

mixture of these two forms. Atomized materials, particularly aluminum, have the. advantage of" high. purityand high reactivity as compared'with flake metals, and

are advantageously employed in sizeswithina range. of.

15-35 microns. Smaller particles. of an average-size of between 5 -15'microns are also suitable.

mas ers: Patented? Aug, 3,., 19.65:

Atomized material is somewhat more susceptible to ignition by electromonoxide as an oxidizing agent.

When flake materials are employed, they are preferably so comminuted that about 90 percent-of the metal will pass a 325 mesh (44 micron) screen.

The metal fuel is employed in combination with lead The lead monoxide oxidizer may be replaced in part with ferric oxide. The inorganic oxidizing materials are preferably finely divided to have a particle size which will pass a 200 mesh (74 microns) screen.

The pyrotechnic compositions of the invention advan-.

tageously contain a minor portion, about 3-20 percent by weight, of the finely divided aluminous metallic fuel, and about 97-80 percent by weight of lead monoxide oxidizer. When the lead monoxide is replaced in part by finely divided ferric oxide, the final composition should contain. about -20 percent by weight of ferric oxide, for best results.

The metal-lead monoxide mixture can also be combined with a synthetic organic resin as earlier mentioned. In particular, the mixture has been advantageously combined with Teflon (polytetrafluoroethylene). The Teflon powder acts both as an oxidizer and a bulking agent, making the pyrotechnic mixture flufly and prevent ing caking or packing due to vibration or handling. It is believed that the electrostatic properties of the polymer also make it hold the finely divided fuel and oxidizer in intimate contact on its surface, promoting their rapid reaction. In such applications, the Teflon is suitably finely divided to have a particle size of about -50 microns, preferably microns.

Suitably, the inorganic mixture can be mixed with finely 'divided polyethylene or polytrifiuorochloroethylene (Kel F) in a similar manner. Also, the inorganic materials may suitably be coated with a thin film of commercial silicone oils by moistening the inorganic mixture with the oils dispersed in a solvent, conveniently hydro-7 carbon materials such as hexane or benzene. On evaporation of the solvent, the oil is deposited on the inorganic particles as a thin film.

When aluminum and lead monoxide mixtures are combined with a synthetic polymer such as polytetrafluoro-, ethylene, a portion of the lead monoxide is replaced by the polymer such that the final composition contains about 8-15 percent by weight of the polymer.

The mixtures found best for use in bridgewire igniters have the following compositions:

Percent by weight Aluminum 3-18 Lead monoxide 82-97 1 When ferric oxide is used in combination with lead monoxide as an oxidizing agent, the preferred compositions are as follows:

Percent by weight The compositions above described have many properties which make them particularly suitable for use in exploding bridgewire igniters. Thus, they are extremely stable in long term storage, even at high temperatures. The individual ingredients are relatively inert. Also, the

' compositions are not prone to detonate when subjected to heat or impact. Finally, the compositions are not'hygroscopic.

In other embodiments, the fuel-oxidizer mixtures, particularly those containing Teflon, can be pressed into pellets or other bulk shapes, with the polymer acting as 4 a lubricant, binder, and oxidizer. Alternatively, the polymer can be used as a binder inpell'et or other compacted forms after moistening with a suitable liquid such as n-hexane. Although such compacted compositions are not specifically adaptable to use in exploding bridgewire igniters, they retain the many safety advantages of the powdered compositions, such as a high ignition temperature, resistance to detonation by impact, etc., and can be employed where ignition is accomplished, for example, by an intense flame from a conventional squib or initiator. The success of a pyrotechnic composition comprising a weak, stable, oxidizing agent such as lead monoxide in combination with an active metal such as aluminum is believed surprising. Specifically, the reaction of aluminum and lead monoxide, with or without the presence of additional oxidizing agents, gives an igniting power" that could not be predicted from considerations of thermal output alone. For example, tests have shown that exploding bridgewire squibs containing a pyrotechnic composition of 12 percent flaked aluminum, 12 percent Teflon, and 76 percent lead monoxide have an ignition delay time of 1 millisecond or less, although they have a heat of explosion of only 478 calories per gram. In com parison, comparable: exploding bridgewire squibs contain ing a conventional mixture of boron and potassium nitrate have an ignition delay time of 5 to 10 milliseconds, al-

though the heat of explosion of this mixture is 1650 calories per gram.

The following specific examples are given by way of illustration, and are not to be construed as limiting on the scope or spirit of the invention.

Example 1 7 Parts by weight Atomized aluminum (5-35 micron) 6.6

polytetrafluoroethylene powder (35 micron) 11.4

Lead monoxide (200 mesh) 82.0

Example 2 Parts by weight Flaked aluminum (325 mesh) 6.6

Powdered polytetrafluoroethylene (35 micron) 11.4

Lead monoxide (200 mesh) 82.0

Example 3 Parts by weight Flaked aluminum (325 mesh) l2 Powdered polytetrafluoroethylene (35 micron) 12 Lead monoxide (200 mesh) 76 Example 4 Parts by weight Atomized aluminum (35 micron) 10.8

Lead monoxide (200 mesh) 89.2

Example 5 Parts by weight Atomizer aluminum (35 micron) 2O Ferric'oxide (200 mesh) 20 Lead monoxide (200 mesh) 60 References Cited by the Examiner UNITED STATES PATENTS 2,900,242 8/59 Williams et al.

2,926,566 3/60 Atkins et al. 89/28 2,953,443 9/60 Lloyd. 2,953,447 9/60 Schulz 149-37 CARL D. QUARFORTH, Primary Examiner.

LEON D. ROSDOL, ROGER L. CAMPBELL,

Examiners.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 3,198,678 August 3, 1965 Samuel Zeman et a1.

Column 2, line 45, for "1.10" read 0.10

Signed and sealed this 12th day of July 1966.

(SEAL) Attest:

ERNEST W. SWIDER Attesting Officer EDWARD J. BRENNER Commissioner of Patents 

